(1) Field of the Invention
This invention is directed to the field of electro-acoustics and more particularly, to a novel dual-frequency sonar system.
(2) Description of the Prior Art
Several patents teach the use of piezoelectric transducers for transmitting or receiving specific frequency bands in underwater applications. One of the earliest is King, U.S. Pat. No. 2,409,632, which employs two forward facing arrays of piezoelectric crystals arranged along either side of a torpedo's forward axis. Each array is comprised of transducers which radiate at and are responsive to a separate frequency band. By turning the torpedo until the received echoes at both frequency bands are roughly equal, the torpedo is able to track and home in on a reflecting target. A drawback of this approach is that the direction of a target may only be determined as left, right or directly ahead. It is necessary that a target be measured with a higher degree of angular resolution. This can be achieved only when numerous beams can be formed.
Hodges et al., U.S. Pat. No. 4,192,246, discloses a torpedo nose design intended to minimize flow noise from sources such as cavitation. The invention includes the use of Tonpilz-type transducers glued to an acoustic window which forms the nose of the torpedo, but enables transmission and reception only at one frequency band.
The use of a composite layered assembly in piezoelectric polymer arrays in hydrophones is disclosed in Francis, U.S. Pat. No. 4,638,468. It teaches the use of a layered assembly of piezoelectric polymer and printed-circuit board material for hydrophone elements and the connection of associated amplifiers. This invention describes a method of constructing piezoelectric polymer transducers, and does not describe a specific application beyond their use in hydrophone arrays. It does not address the possibility of using multiple arrays to operate at more than one frequency band, the same drawback found in Hodges et al.
Hoering, U.S. Pat. No. 4,916,675, proposes a method of using unique transducer rings to form a device which can radiate and receive more than one range of frequencies. This approach, however, provides radiation and reception of a full three hundred-sixty degrees. The ability to form numerous beams for the determination of angular offset is missing.
A simple, low cost and easily fabricated arrangement of piezoelectric transducers with maximized response characteristics in more than one frequency band along the axis of an underwater vehicle is lacking, and much needed, to provide effective and timely detection and identification of underwater objects.